1. Field of the Invention
The present invention relates to a method and an apparatus for regulating state of charge, and more particularly, to the method and the apparatus for regulating the state of charge in a battery assembly which includes a plurality of secondary cells as unit cells connected in series.
2. Description of the Related Art
In an electric car driven by an electric motor and in a hybrid electric car driven by both an engine and an electric motor, a battery assembly including a plurality of secondary cells as unit cells connected in series, such as a nickel-metal hydride battery, a lithium battery, has been employed as a power source for the electric motor.
It has been known that the above described battery assembly has had such a problem that due to repeated charging and discharging of electricity, variations may occur in respective voltages across the cells depending on the state of charge (SOC) of the unit cells, and in case where charging and discharging of electricity are repeated while the variations remain unsolved, some of the unit cells may fall in an excessively charged state or in an excessively discharged state.
Under the circumstances, it is proposed, in Japanese Patent Laid-open Publication No.10-225005, that the unit cells are respectively connected to a capacitor having a determined capacitor voltage. In this structure, electric charge of the unit cell which has a higher voltage across the cell than the capacitor voltage is transferred to the capacitor, and contrarily, the capacitor voltage is transferred to the unit cell which has a lower voltage across the cell than the capacitor voltage. In other words, the stored electric charge is transferred from the unit cell having the higher voltage across the cell to the unit cell having the lower voltage across the cell by way of the capacitor, and consequently, the variations in the respective voltages across the unit cells can be eliminated.
However, according to the method proposed in Japanese Patent Laid-open Publication No. 10-225005, when the respective voltages across the unit cells have been equalized and the variations in the respective voltages across the unit cells have been minimized, differences between the capacitor voltage and the respective voltages across the unit cells will be decreased. As the results, an amount of the electric charge to be transferred will be decreased, and it will take a considerable time to completely equalize the voltages across the unit cells.
In view of the above described problems, an object of the present invention is to provide a method and an apparatus for regulating state of charge in a battery assembly in which respective voltages across unit cells constituting the battery assembly can be equalized in a short time.